1. Technical Field
The present invention relates to semiconductor diodes, and more particularly, to tunable semiconductor diodes.
2. Related Art
ESD (electrostatic discharges) diodes are well known in present day semiconductor technology in order to protect I/O and other internal circuitry from electrostatic discharges and other overvoltage conditions that could cause catastrophic failure in integrated circuits. See e.g. U.S. Pat. No. 5,629,544 to Voldman et al., “Semiconductor Diode with Silicide Films and Trench Isolation,” which teaches ESD diodes having silicided electrodes to reduce series resistance, and U.S. Pat. No. 5,945,713 to Voldman, “Electrostatic Discharge Protection Circuits for Mixed Voltage Interface and Multi-Rail Disconnected Power Grid Applications,” which teaches an ESD diode that can interface with multiple supply voltages.
Because of high sheet resistance, these ESD diodes must be made large in order to discharge the ESD currents. Voldman et al using a p+/n well diode showed that ESD improvement can be achieved by reduction of the n well sheet resistance in a CMOS technology from 1000 Ohms/square to 300 Ohms/square. In a CMOS technology, this also leads to an increase in the capacitance per unit area which is acceptable for CMOS technologies below 1 GHz application frequencies but not for application speeds above this frequency range. Large ESD structures are also unacceptable for high speed RF (Radio Frequency) applications due to the high capacitance load placed on the RF circuit.
As integrated circuit (IC) switching speeds approach and pass 1 GHz, there is a need to reduce the capacitance of ESD diodes. This is because as switching speeds increase, device area decreases, such that device reactance remains relatively constant. As capacitance decreases, it is desirable to avoid compromising the ESD robustness (that is, the breakdown voltage) of the diode. In general, these goals have been mutually exclusive; ESD robustness is typically preserved by making the diode structure fairly large, which drives up capacitance. Thus, there is a need in the art for an ESD diode that has reduced capacitance without reduced ESD robustness.